Deck Height Variance

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sbh126

sbh126

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How much varience is normal between cylinders regaurding deck height? My #1 cylinder measures at .012 in the hole at the top of the piston and .022 on the bottom of the piston face. My # 7 Piston measures at .006 in the hole on the top and .012 in the hole at the bottom half of the pistons face. Is this something to be worried about?
 
77' 440 block with 68' forged crank, stock 77' rods and brand new KB237 .030 pistons.
 
Deck height is one of the worst things Chrysler ignored. It is normally all over the place. Yours actually sounds like a good example. It would be well worth your time to have the deck height blueprinted.
 
What SS said. It's very common for the older millers to not be able to cut it very flat in comparison with new stuff. It's because of the cutting wheel and how the blocks are leveled before they are cut. The best thing you can do is have it square decked, which indexes the deck surface off the crank and cam bore centerlines, and have it cut to the height you need it to be.
 
Have the decks cut to the same height, as previously mentioned, deck hight variance was terrible, but as long as you're doing that, it might not be a bad idea to have the rods measured too, and resized if needed.
 
My brother and I have just dummied up a 383 with 440 forged crank bored 60(438 inch stroker) to check for deck height and squareness.New pistons and rods.
We measured top,bottom and ends on cylinders 1,2,7 and 8.
We were very surprised to find max variance of only 4 thou.
I guess ours is one of the better ones.
It's a 1965 HP block if that matters.
 
I'll take it in and have it decked to clean it up a bit. I had my rods resized recently so they ought to be good. What's a general idea on how much it should cost to get it decked? Anything special to ask for when I take it in?
 
sbh126 said:
I'll take it in and have it decked to clean it up a bit. I had my rods resized recently so they ought to be good. What's a general idea on how much it should cost to get it decked? Anything special to ask for when I take it in?
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I think that depends on how much they have to take off. I know when I started pricing machine work. Seemed like it revolved around .010 increments.
 
Square decking costs me $400, with no limit on material removed and any finish I want. Decking, and square decking, are not the same thing.
 
moper said:
Square decking costs me $400, with no limit on material removed and any finish I want. Decking, and square decking, are not the same thing.
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What exactly is the difference? I'd like to know what I'm asking for when I go to the machine shop.
 
The machine shop I used lined honed the mains first then used the crank center line to deck the block to put the pistons at zero deck. Not every maching shop has equipment to do it that way.
 
sbh126 said:
What exactly is the difference? I'd like to know what I'm asking for when I go to the machine shop.
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Your standard old-school miller consists of an anchoring system for the block either on a table or in an enclosure, and uses a round cutting head with a series of cutters around the perimeter. The block is placed in it and bolted down. Normally the main caps are off, and there's a bar that sits in the main saddles, or it could be sitting on a table and it's leveled at this point by eye using a machinist's level to the table. The diameter of the cutting wheel is important, and the bigger the better, because the cutting head is angled slightly so only one edge of the head contacts the deck at a time. A smaller wheel's teeth will dig deeper in the center of the cut as a result of that tilt... Making the area between the cylinders a low spot. This equipment can machine the deck surfaces resonably flat, givent he guy has a good eye, and the level hasn't been dropped, and the main saddles are machined properly. But there is no guarantee that they are exactly parallel to the crank's centerline or perpendicular to the bore centerline. There are fixtures that can be bought that will use a heavy plate (sometimes called a BHJ plate for one of the manufacturers) and a series of bars to align the block and index the deck cutting to the crank and camshaft centerlines. That fixture allows the decks to be cut square. Hence the term "square decking". In my opinion, a modern performance shop does not use a miller with the multiple cutter head. They use a machining center (like Rottler or Sunnen) where the feed rate and cutter speed can be adjusted, and because it's a single cutter, there is no tilt, and so no low spot. The modern equipment also makes setting a specific deck height MUCH easier because the cutting head is controlled better and it's position in space is known by a readout. So the machinist can simply see where the crank centerline is, and calculate exactly where the head will need to be to give the desired deck height. It's also very easy to get the right gasket finishes for different head gaskets. Most of the old millers have a hard time getting things as smooth as the MLS gaskets need.
If your shop does not have the ability to square deck, find another shop. That plate, even without the big CNC machining centers, is a small investment for a performance shop that gives immediate and tangible results. If possible, find a shop that uses the machining center. They are aways more expensive. But square decking is a MUST when building for quench.
 
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